The jet turbine engine which powers modern aircraft is a device which relies for its operation on creating and utilizing high pressure gas streams. A compressor section, at the inlet of the engine, compresses external air, creating hot high pressure air. Most of that hot high pressure air is mixed with fuel in a combustor and burned, thus creating energy for propelling the aircraft. Some of the high pressure air can also be bled off for other purposes, such as cabin pressurization and anti-icing of leading edges. To meet these requirements, the modern jet turbine engine will typically be found to have an array of ductwork or manifolds which conduct the high pressure air to the locations where it is required.
The temperature of the gas can reach 1000.degree. F. or more, and manifold or duct failures are not uncommon. A failure in the manifold or duct can also damage other nearby sheet metal or components. The engine itself is usually encased in a shroud, and sometimes located in a relatively complex compartment in the tail of the aircraft or in a pylon mount. Thus, it is not an easy matter to visually inspect the engine and its ductwork for evidence of gas leakage. Systems have therefore been devised for monitoring the areas around or near the gas turbine engine to sense and enunciate the occurrence of any leaks. It is also an aid to the flight crew to have some information on the location of the leak. For example, if the tail engine has developed a leak in the manifold, it is useful to know whether it is in the left or right side manifold, and action can be taken by the flight crew to continue to utilize the engine while avoiding pumping of hot air through the failed manifold section. It is also a benefit to the ground maintenance crew to be able to interrogate a monitoring system to determine with greater precision the location of any leaks, so that repairs and inspections can be made in an expeditious manner. To simply know that a leak has occurred, without more, might cause the ground crew to completely inspect extensive areas on and around the engine. With an automatic monitoring system, however, if the system were capable of reliably identifying the approximate position of all sensed leaks, maintenance personnel would then only need to inspect and repair those portions of the ducting which were known to have failed.
Systems have been proposed in the past for performing remote monitoring. Long cable-like sensors are associated with the manifold, and monitored by an automatic system in an effort to accomplish these goals. One type of elongate cable sensor is of the thermistor variety where a coaxial metallic cable of substantially zero resistance has a thermistor-type insulant separating a central conductor from an outer jacket. When the cable is heated over a reasonable portion thereof, the thermistor-type insulant material changes characteristics indicating the fact of a failure. The location of the failure is not quite so readily indicatable, however. A further limitation with this type of device is the fact that a rather large segment of the cable must be heated in order for the thermistor-type insulant material to perform its function, making it difficult to identify or pinpoint small localized leaks.
It has also been proposed to use a coaxial cable, which is like the thermistor cable in that both the jacket and shield have essentially zero resistance, but the separator material has a capacitance which changes after it is heated to a predetermined temperature. A capacitance bridge can monitor this type of cable and attempt to determine the position of localized heated portions of the cable. However, such an approach suffers from the difficulties of operating a capacitive bridge, including the interference effects which can arise from electromagnetic interference generated by other equipment in the aircraft. Another potential problem is electromagnetic interference generated by the system itself in operating the capacitance bridge at frequencies adequate to perform the necessary sensing.
Thus, while systems have been proposed which are theoretically capable of performing the monitoring function for locating localized leaks in aircraft pneumatic manifolds, those systems and their associated sensing cables have not been entirely satisfactory.